The invention relates to an X-ray source for ionizing of gases, with a field emission tip array in a vacuum chamber.
Such X-ray sources for ionizing of gases consist of multiple individual X-ray sources arranged lengthwise in a row by means of a bar-shaped carrier and supplied with a required voltage. The X-ray sources are mounted here in an air flow in such a manner that they ionize the surrounding air of the bar-shaped arrangement. The air that has been ionized in this manner can be used for the neutralization of electrostatic charges in critical fabrication areas.
For example, such X-ray sources can be inserted in the usually vertical air flow in the clean room of fabrication installations in the microelectronics industry, in order to neutralize electrostatic charges on objects by means of the ionized gases. This is necessary because electrostatic charges lead to increased particle deposition. On the other hand, uncontrolled discharges can cause damage to the microstructures. The electrostatic discharges are referred to as ESD (ElectroStatic Discharge). Ion generators are also used for generating a predetermined ion density of differently charged ions within a cabin or other space, wherein the recombination of the ions usually starts immediately after they have been generated.
The commonly used ionization systems generate the ions by means of a high voltage at tip electrodes via gas discharge. In the process, both direct current voltage and also alternating current voltage, or so-called pulsed direct current voltage, are used. Thus, different methods are used in order to minimize the recombination of positive and negative ions. An essential aspect of these ionizers is the achievement of a balanced ratio of positive and negative ions in order to ensure complete neutralization of the charges.
Thus, DE 600 34 040 T2 discloses an ion generator device, in which several upright emission needles next to one another made of titanium, platinum, a titanium-platinum compound or made of a rust-free metal, or of different alloys are used. The emission needles consist of a cylindrical core which is enclosed by a composite material made of an unsaturated polyester compound containing glass fibers. Each needle tip ends outside of a housing in a conical indentation which can be covered by a grate. At the needle tip, a strong voltage field or a potential difference along the needle is generated in order to ensure the generation of electrons by the emission tips. For this purpose, the needles are directly connected to a high-voltage source with a high voltage of 4.3-6 kV. The needle tips can additionally be coated with a gold film.
Moreover, DE 691 11 651 T2 discloses a clean-room corona air ionizer which is provided with at least one corona tip connected to a high-voltage source. The corona tip is located within a tube which is closed off at one end and an anhydrous hydrogen-rich gas flows around the tip. This corona air ionizer enables the removal of microcontaminations such as ammonium nitrate accumulations which can form in corona ionizers.
In DE 698 18 364 T2, germanium emitting electrodes are used, in order to reduce metal contaminations during the production of integrated circuits to the lowest possible level. The emitter electrode has a tip which ends with a conical radius. The germanium emitters are semiconducting and have a specific resistance of approximately 0.1-100 Ωcm. In order to achieve this, the germanium emitters are preferably doped with antimony.
U.S. Pat. No. 5,729,583 relates to a miniature low-energy X-ray source for diagnostic radiography with an emission tip array arranged in a vacuum chamber on a cathode surface, and wherein a flat gate arrangement is provided above the emission tip array. The associated anode in the form of a metal film extends in the vacuum chamber along the inner wall and in front of a collimator before the emission tip array. The emission tips are located in cylindrical openings of the flat gate arrangement, and an insulation layer extends between the cathode surface and the gate arrangement.
A similar construction for an emission tip array is disclosed in U.S. Pat. No. 3,665,241.
DE 10 2009 031 985 A1 discloses an ionizer with multiple electrode pairs having different electrode lengths arranged on the periphery of a circle and protruding into an air flow, in which the positive and negative ion flows do not overlap. In this manner, the number of ions neutralized by new recombination is reduced, so that the discharge efficiency on the work piece to be protected is improved.
However, in most cases, the systems used are in the shape of bars, wherein, on the lower side, the emitter tips integrated in the high-voltage lead protrude outward.
U.S. Pat. No. 6,807,044 B1 discloses a typical example of such a system. Here, a support bar in the shape of a bar supports a plurality of pointed emitter electrodes next to one another, each of which is accommodated within a tube-shaped sheathing that is open at one end. The support bar is enclosed by a housing, in which the required power supply is accommodated.
Similar bar-shaped ionization devices are also described in WO 2009/031764 A2 and in JP 2010 218696 A.
Such ionization devices are commonly arranged in an air flow generated by an ultra-clean air installation and directed onto the work pieces to be protected, such as semiconductor wafers or other sensitive fabrication devices.
Such gas discharge ionization systems operated at high voltage have a number of disadvantages. Thus, the balance between the generated negative and positive ions depends very strongly on the high-voltage power supply and the geometric arrangement of the emitter electrodes. I.e., pulsed or alternating high-voltage systems exist, in which positive and negative ions are generated successively in a temporal sequence.
Direct current voltage systems have spatially offset emitter electrodes for positive and negative high-voltage. Thereby, the respective polar ions are generated at different sites, which in turn leads to imbalances in the polarity distribution and, as a result, to charging processes in the area of influence. These processes are site-dependent charging processes in each case.
Moreover, the long-term polarity balance is unstable, since the emitter electrodes change over the course of time. The high-voltage ionizers consequently have to be checked and readjusted constantly.
In addition to the influence due to different ion concentrations, there are also undesired influences of the ion-generating high voltage on the emitters, which frequently exceeds 15 kV.
In particular, in installation situations in which the products to be neutralized come close to the ionizers, considerable potential differences are sometimes generated due to the electrical field influence.
In addition, on the emitter electrodes themselves, under ultra-clean air conditions, particulate contaminations form due to electrochemical conversion processes, which impair quality in the ultra-clean fabrications of the microstructure industry. The emitters of high-voltage ionizers therefore have to be cleaned regularly, which is associated with an interruption of the production sequence.
With increasingly smaller structures, the sensitivities of the substrates to electrostatic charges continue to increase. With the structure sizes of today, the limitations of the high-voltage ionizers used are obvious and other options are sought.
Thus, ionization systems which generate air ions by means of low-energy X-ray radiation are commercially available. Here, an energy range below 5 keV is used increasingly. These systems in which X-ray tubes are used have the advantage that they have no electric field effect toward the outside and the ion polarity balance is always perfectly equilibrated. This is due to the fact that the ions in the air volume are generated by means of ionizing radiation by cleaving off an electron and the associated remaining of the positive ion core. Beyond this, no contaminations are formed.
The disadvantage of this system is, on the one hand, the relatively high price, and, on the other hand, the low useful life of the X-ray tubes used. In the currently used systems, X-ray tubes with incandescent cathodes are utilized, which, as a rule, have a useful life of approximately one year. This means that the X-ray tubes have to be replaced annually, which is time consuming and expensive.
An additional disadvantage of these systems consists in that they are quite large and therefore often difficult to integrate in installations, and, in addition, they contain the relatively expensive control electronics and monitoring needed for incandescent cathodes.
An example of such an in-line gas ionizer is disclosed in WO 01/84683 A2. This ionizer comprises pressurized air sources with an air channel for the fabrication device and an X-ray radiation source integrated in the air channel.